use ob_get_clean() instead of ob_get_contents() to avoid double rendering of...

use ob_get_clean() instead of ob_get_contents() to avoid double rendering of plugin contents, fixes #5062

Closes #5062

Merge request studip/studip!3784

lib/plugins/core/StudIPPlugin.php

@@ -225,7 +225,7 @@ abstract class StudIPPlugin
             return function ($request, $response, array $otherargs) use ($action, $args, $that) {
                 ob_start();
                 call_user_func_array([$that, $action], $args);
-                $content = ob_get_contents();
+                $content = ob_get_clean();
                 $response->getBody()->write($content);
                 return $response;
             };